Transforming Growth Factors (TGFs) were isolated from media of human tumor cell cultures and extracts of the tumor cells themselves. The TGFs, including Sarcoma Growth Factor were purified, characterized and their interaction with cell membrane receptors studied. Based on these studies the "autocrine theory" of cell self-stimulation was proposed. TGFs appear during embryonic development and are detected in the urine of pregnant humans and of a patient with small cell lung carcinoma. In addition, a variant of Epidermal Growth Factor (EGF) was detected in urine from normal humans. Human TGFs are closely associated with tyrosine-specific protein kinase activity, while EGF receptors appear to be the substrate for phosphorylation of tyrosine acceptor sites. Immunosuppressive factors (ISF) were identified in sarcoma virus-transformed mouse cells and in human bronchogenic carcinoma cells. A low molecular weight peptide was isolated from crude preparations of interferon that had no interferon activity but inhibited the growth of human melanoma cells. Cells from human lung cancers were examined for specific membrane receptors for EGF and nerve growth factor. Based on the pattern of presence and absence of these receptors, fundamental biologic, and possibly diagnostic, classification of these tumors are seen. The primate type C viruses CPC-1 and MAC-1 were further characterized and classified on the basis of their major internal protein and terminal nucleotide sequences.